But the question remains how does something like this happen at an army base? More specifically, at this army base? We're joined now by ABC's chief global affairs correspondent who has recorded extensively on units and families at ft. Hood. This is a community. They used to call ft. Hood in 2004 pleasantville. It is kids riding up and down the streets on their bicycle. One of the signs when the men are overseas when they're deployed, men and women, is the lone vehicles in the driveway, the empty truck. Very poignant scenes when you dro drive around that army post when people are deployed. And they're deployed all the time, Dan. These are two divisions within ft. Hood, the cavalry division which I have covered for well over a decade and the fourth infantry division. Those two divisions make up about 40,000 people there or many and their families live there on base. When you talk about what happened today, and you can hear people say it was miles away or it was just down the street, they are bowling alleys, they are restaurants, there are kids going to school every day. So you really have to think about this as a community. So many soldiers here in ft. Hood. And many just returned in Afghanistan, correct? Many just returned from Afghanistan, indeed, including lieutenant general mark milli who you saw tonight give that press conference. I have spent so much time with lieutenant general milli over the years. He's someone I know well and this has to be devastating to him, to his family, to that community in ft. Hood. I think many will hear that it happens in an army base and say how can someone with a gun do this kind of damage at an army base, but there are actually rules there with regard to posessing weapons at ft. Hood. Good point, Dan. Because I think that's also one of those images people think, well, you're going to an army post, an army base. So there must be people walking around with guns. They aren't. Again, it's a community. They have to register those weapons and they are not supposed to be walking around that base with personal weapons concealed on that base. So the military police officer who confronted the shooter was armed, but he certainly shouldn't have been. In fact, I guess ran back out to his vehicle to get the gun and then went back in and started shooting. Now, Martha, we know that Ivan Lopez was being treated for mental health problems and it turns out koint dentally, you were at the Feng today talking about post traumatic stress disorder, what do they have to say? Really quite an extraordinary coincidence, Dan. I was at the Pentagon talking to the deputy commanding general of army medical command and we were talking about post traumatic stress syndrome and we were talking about how they diagnosed that and how hard that is to diagnose. And he said well, actually, we still work on this. We don't know exactly how to approach this. But this is an enormous problem in the U.S. Military. There's 1 in 5 reporting mental health issues or traumatic brain injury. We have been deployed in this country, we sent our troops over there for well over a decade and they have many of them come back with issues like this. But we learned today that Ivan Lopez has not officially been diagnosed with post traumatic stress disorder E, correct? That's correct. Lieutenant general milli said they had not officially diagnosed him, but he was undergoing a diagnosis. I think it takes a long time to diagnose someone whether they have this post traumatic stress disorder, as they call it. But he was under medical care. He was on medication, they said, for depression for anxiety and he was taking medication for that. Thank you very much. We appreciate it.

This transcript has been automatically generated and may not be 100% accurate.

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